Interpretive Summary: Chlorophyll meters provide a fast and reliable indication of crop nitrogen (N) status, but application of this technology on a whole field basis would be labor intensive and time consuming. Corn leaves from four hybrids growing at five rates of N fertilizer were evaluated for N status using the hand-held Minolta chlorophyll meter and also using a laboratory instrument. Chlorophyll meters operate by measuring the amount of red light passing through a leaf that is not absorbed by chlorophyll. In contrast, the laboratory instrument measures the amount of light reflected from a leaf surface, but does so for a wide range of visible colors. The objective of the research was to compare the results using the two techniques and determine the wavelength of reflected light that was best correlated with the chlorophyll meter readings. Results showed that green light with a wavelength of 500 nm was the most highly correlated with leaf N concentration and chlorophyll meter readings. These results suggest that remote sensing techniques that measure green light reflected from a crop canopy should be able to monitor crop N status.

Technical Abstract:
New tools that can rapidly quantify the N status of corn could be valuable in N fertilizer management practices. This study was conducted to compare light reflectance from corn leaves with other parameters used to detect N deficiencies. Light reflectance (400-700 nm) as measured from corn leaves in the laboratory with a Hunter tristimulus colorimeter was compared with Minolta SPAD 502 chlorophyll meter readings (light transmittance at 650 and 940 nm), leaf N concentrations, and specific leaf N (N content per unit area). Measurements were made on individual ear leaves collected from an irrigated corn N response trail with four hybrids and five N treatments. Light reflectance near 550 nm was the best wavelenth to separate N treatment differences. Reflectance at 550 nm provided a stronger relationship with both leaf N concentration and chlorophyll meter readings that between chlorophyll meter readings and leaf N concentration. The measurement of light reflectance near 550 nm has promise as a technique to detect N deficiencies in corn leaves.